Add python version compatibility tests

[JasonMoho]

Is your feature request related to a problem? Please describe.
We previously have observed occasional issues when using Python 3.8 and 3.9, but that behavior wasn't documented.

A user recently reported that they had trouble using python 3.8 and 3.9 with the system. #55 (comment)

Describe the solution you'd like
The tox test suites should be modified to run under python 3.6-3.9.
We should document and fix any compatibility issues with specific python versions.